This invention relates to seismic exploration and more particularly to the stacking of seismograms to suppress noise and enhance the reflections on the seismograms.
In seismic exploration, seismic energy is generated by a source located at or near the surface of the earth, is reflected from subsurface interfaces between layers of the earth, and is received by a spread of seismic detectors having different offsets from the source on the surface of the earth. The detected seismograms are gathered, stacked, and recorded in the form of a set of common depth point (CDP) seismic traces as illustrated, for example, in FIG. 1 of U.S. Pat. No. 4,206,509--Ruehle. From these recorded CDP seismic traces the subsurface interval velocities can be determined. Such a CDP gathering and recording technique and a continuous velocity determination technique are disclosed in U.S. Pat. No. 3,651,451--Ruehle. An accurate velocity determination is quite important in the determination of the nature and extent of the subsurface layering.
It has long been recognized that not all field traces should be included in the stack. Some of the field traces have particularly high noise levels. These field traces are muted before the field records are gathered and stacked. The conventional way of determining which traces should be muted is by shooting a noise spread in the field. The seismograms from the noise spread define the noise and signal properties for each field array. Large amplitude noise, predominantly multiple refractions, surface waves, and multiple reflected shear refractions often interfere with the reflections on certain traces. These noise arrivals and the amplitude of the desired reflections, are determined from the seismograms from the noise spread. From the noise and reflection amplitudes, the ratio of reflection to noise amplitude as a function of time and offset, is determined. This ratio is used in a weighting matrix or selection matrix which selects the time offset continuation of field records which will best stack the traces. The noisy traces are muted and the remaining traces are stacked to obtain seismic sections with good signal to noise ratios. An interpreter analyzes the records from the field spread and picks the optimum time velocity offset function for stacking. Alternatively, procedures are available for automatically selecting this function. Coffeen, "Seismic Exploration Fundamentals", pp. 108-110, Petroleum Publishing Company, Tulsa, Okla. describes muting traces prior to stacking.
In accordance with the present invention, the velocity offset as a function of time is determined from the field traces in order to stack those traces with the highest signal to noise ratio.